No. Boat insurance is not a legal requirement in the UK but can be in other countries. Even within the UK certain authorities may require you to hold at least Third Party Only insurance to access rivers, canals boatyards or marinas.
The Canal and River Trust, which issues licences to use the majority of Britain’s waterways, specifies that boaters will need third-party cover for at least £2m and the Environment Agency, which deals with licences for the Thames, asks for at least £1m of third party cover.
Most policies will include adequate third-party cover, but check the requirements of your boat licence issuer to make sure you choose a policy that includes enough protection.